Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image, the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally; a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a fixing roller heated by a heater and a pressing roller pressed against the fixing roller to form a fixing nip therebetween through which the recording medium bearing the unfixed toner image is conveyed. As the recording medium passes through the fixing nip in a state in which the front side of the recording medium that bears the unfixed toner image contacts the fixing roller, the fixing roller heated by the heater and the pressing roller apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium. In duplex printing, the recording medium is reversed after it is discharged from the fixing device and then conveyed through the fixing nip again in a state in which the back side of the recording medium that bears the unfixed toner image contacts the fixing roller and the front side of the recording medium that bears the fixed toner image contacts the pressing roller. Thus, the fixing roller and the pressing roller fix the toner image on the back side of the recording medium.
In duplex printing, it is important to prevent overheating of the pressing roller, which may cause failures described below. For example, if the surface temperature of the pressing roller is excessively higher than the surface temperature of the fixing roller, the gloss level of the toner image formed on the front side of the recording medium may be different from the gloss level of the toner image formed on the back side of the recording medium, minute scratches on the surface of the pressing roller may damage the toner image formed on the recording medium, or the recording medium may not separate from the pressing roller readily when it is discharged from the fixing nip. These failures are conspicuous when glossy paper or coated paper in increasing demand is used as the recording medium.
To address this circumstance, the fixing device may incorporate a fan that produces airflow inside a housing of the fixing device, which blows air on the surface of the pressing roller to cool it. However, airflow may also impinge on a temperature detector that should be protected against airflow to detect the surface temperature of the pressing roller precisely, resulting in erroneous detection and malfunction of the temperature detector.
Alternatively, the fan may blow air on the pressing roller through a duct. However, air discharged from an outlet of the duct may be directed to the temperature detector by the pressing roller upon impingement on the pressing roller, resulting in erroneous detection and malfunction of the temperature detector.